C-type lectins constitute a key class of pathogen receptors on dendritic cells (DCs), one of the main antigen presenting cell types in the immune system. One such receptor, DC-SIGN, binds to a large variety of pathogens, including dengue virus, HIV and yeasts, by recognizing carbohydrates on the pathogen surfaces. DC-SIGN is found assembled in small domains on the plasma membranes of DCs, and this clustering is required for DC-SIGN to successfully bind to and internalize pathogens. These domains possess some striking properties. First, the DC-SIGN molecules appear to be immobilized within the domains. Second, the origin of this immobilization is initiated in the extracellular carbohydrate recognition moiety rather than the membrane apposed cytoskeleton. Third, super-resolution microscopy shows that the microdomains, as imaged by conventional fluorescence microscopy, are actually composed of arrays of ~75nm nanodomains containing only about 3 tetramers of DC-SIGN, which indicates the presence of other proteins and lipids within these nanodomains. To elucidate how these nanodomains carry out their function in DCs, we will define their nanoscale properties after pathogens are bound by using super- resolution microscopies and, furthermore, investigate the mechanisms of transport from binding to internalization sites. We will also define the at present unknown nanodomain protein composition using modern proteomic technologies complemented by state of the art domain isolation methods. Information on this level of detail will not only provide an increased understanding of membrane domains in general, but will also provide a much more complete picture of the initial recognition and processing of infectious agents by the human immune system.